1. Field of the Invention
This invention relates generally to a system that converts an analog signal to a digital signal having a lower frequency representation and, more particularly, to an oscillator/counter analog-to-digital converter that simultaneously performs frequency downconversion, band pass filtering and an analog-to-digital conversion of an analog signal using a superconducting, Josephson junction single flux quantum circuit to extract information from a modulated carrier wave in a communications, radar, or other system.
2. Discussion of the Related Art
Various communication systems, such as cellular telephone systems, radar systems, etc., transmit information by modulating a high frequency carrier signal with the information to be transmitted. Different modulation techniques are known in the art, such as amplitude modulation, frequency modulation, phase modulation, etc., that impress information onto a carrier signal to be transmitted. The carrier signal is received by a receiver that removes the carrier signal to separate and decipher the transmitted information. To remove the carrier signal, state of the art receivers typically include an analog mixer or a frequency downconverter that multiplies the received carrier signal with a local oscillator signal to remove the carrier signal and convert the signal to a lower intermediate or baseband frequency. The downconverted frequency signal is then filtered by a pass band filter that passes the frequencies of interest that include the extracted information. The filtered signal is then converted to a digital signal by an analog-to-digital (A/D) converter to provide a digital representation of the information that is subsequently processed by a microprocessor. This general description of the process for extracting information from a carrier signal is well known to those skilled in the art.
Although this type of circuit is successful for extracting transmitted information from a carrier signal, improvements can be made. For example, because these types of communication systems first mix the analog carrier signal to provide the downconversion and then filter the downconverted analog signal before the signal is converted to a digital representation, noise from the various amplifiers and other electrical components in the downconverter and filter decreases the signal-to-noise ratio of the signal and thus degrade the receiver performance. Additionally, it takes several discrete electrical circuits to perform the mixing, filtering and analog-to-digital conversion. Therefore, the communication electronics could benefit from decreased complexity, part count, and power consumption of these circuits.
Alternately, frequency downconversion can be performed digitally. A straight-forward method of digitally performing frequency downconversion is to digitize the carrier signal fast enough to record the carrier directly. In principle, the information on the carrier signal can be extracted from the digital data stream using fast Fourier transform (FFT) routines and other digital signal processing techniques. This type of method stresses the performance of the A/D converter and the digital processor, because it needs to sample the signal fast enough to record the carrier while maintaining a very high dynamic range to avoid degrading the signal and the information bandwidth. Because of this requirement, these systems would require an A/D converter and digital signal processor performance which cannot yet be realized in the state of the art.
A second digital technique, presently used to effectively produce frequency downconversion, is known as intermediate frequency (IF) sampling. In IF sampling a narrow band pass analog filter, centered at the carrier frequency, precedes a standard non-integrating A/D converter. The A/D converter is intentionally operated well below the Nyquist condition for the input signal, generating an alias of the signal which effectively converts the frequency of the information. The presence of the narrow band pass filter removes the ambiguity in original signal frequency usually introduced by aliasing in A/D conversion. This technique is fundamentally different from the present invention. IF sampling is based on instantaneous samples of the signal where the sampling is done on a time scale very short compared to one period of the carrier signal. The present invention is based on an integration of the signal over a time longer than a few periods of the carrier signal. This difference leads to significantly different requirements for the analog signal filter and leads to the much greater flexibility of the present invention.
Oscillator/counter A/D converters that use superconducting, Josephson single flux quantum (SFQ) circuits for converting an analog signal to a digital signal are known in the art. See, for example, L. R. Eaton, et al., "Design of a 10 K NbN A/D Converter for IR Focal Plane Array Sensors," IEEE Transactions on Applied Superconductivity, 5, 2457, (1995). An improvement to the oscillator/counter A/D converter architecture of the type disclosed in the L. R. Eaton et al. article can be found in U.S. Pat. No. 5,942,997, titled Correlated Superconductor Single Flux Quantum Analog-to-Digital Converter, assigned to the assignee of this application, and herein incorporated by reference.
A general depiction of an oscillator/counter A/D converter 10 of the type disclosed in patent application Ser. No. 08/920741 is shown in FIG. 1. The converter 10 includes a voltage controlled oscillator (VCO) 12, a digital gate circuit 14 and a digital pulse counter circuit 16. Each of the VCO 12, the gate circuit 14 and the counter circuit 16 are general representations of known electrical circuits that perform the functions described herein. The analog carrier signal is received by an antenna (not shown) and is applied to the VCO 12. The VCO 12 converts the analog signal to a series of high frequency SFQ pulses having a pulse frequency proportional to the voltage potential of the analog signal applied to the VCO 12. The VCO 12 uses multiple Josephson Junctions within a direct current superconducting quantum interface device (SQUID) to convert the analog signal to the series of SFQ pulses. The repetition rate of the pulses from the VCO 12 is dependent on the amplitude of the carrier signal and the information modulated thereon. In other words, the VCO 12 will output the pulses at a certain pulse rate depending on the characteristics of the modulated carrier signal. Typically, the pulse rate of the output of the VCO 12 will be much greater than the frequency of the carrier signal.
A control signal is applied to the gate circuit 14 such that when the control signal is high, the gate circuit 14 will pass the pulses from the VCO 12. When the gate circuit 14 passes the pulses from the VCO 12, the counter circuit 16 accumulates and counts the pulses to give a digital representation of the analog input signal to the VCO 12. In one embodiment, the counter circuit 16 is a single flux quantum counter comprising a chain of flip-flops which operate asynchronously to accumulate the total number of pulses from the VCO 12. The total count of the pulses from the VCO 12 during the time that the control signal to the gate circuit 14 is high is the digital representation of the analog signal integrated over the sample time. The state-of-the-art oscillator/counter A/D converter resets the counter circuit 16 to zero before each sample time. In other words, each time the control signal applied to the gate circuit 14 goes low, the counter circuit 16 is reset, so that the sample period is the length of time that one gate pulse is high.
It is an object of the present invention to control the operation of the oscillator/counter A/D converter 10 so that not only does the converter 10 give a digital representation of the analog input signal, but simultaneously performs frequency downconversion and band pass filtering of the signal. Such a control scheme can be used to replace the existing circuitry in communication systems that perform frequency downconversion, low pass filtering and analog-to-digital conversion, and thus, improve performance, decrease complexity, part count and power consumption, and increase the flexibility of the downconversion function.